The use of multimode slab waveguides where overlay devices are used to perform geometrical optics on the propagating light is restricted because device operation is mode dependent. When multiple modes are excited within a waveguide, beam steering or focusing properties can vary significantly. This is a major concern where there is an optical axis along which there are additional devices having dimensions about the size of the beam, which typically occurs.
There are, however, several advantages of using waveguides supporting multiple modes. Often a thin-film preparation technique, required for some materials and their associated characteristics, will limit the thinness of resulting films and so result in the appearance of multiple propagation modes. These methods might include: flame hydrolysis of silicon dioxide doped films, dip-coating from sol-gel solutions or indiffusion of dopants into existing waveguides or substrates. It is desirable to have a waveguide structure that can operate with a thicker thin-film and take advantage of the benefits of thicker waveguide layers. Two of the benefits of using thicker waveguide layers are lower propagation losses and more efficient end-fire coupling of light from a fiber, laser diode, or microscope objective.